1. Field of the Invention
The present invention relates to a method and apparatus utilized in a wireless communication system, and more particularly, to a method and apparatus of enhancing performance of downlink multi-user multiple-input-multiple-output transmissions in a wireless communication system.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3GPP Rel-8 standard and/or the 3GPP Rel-9 standard are developed by the 3rd Generation Partnership Project (3GPP) as a successor of a universal mobile telecommunication system (UMTS) for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple UEs, and communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint transmission/reception (CoMP), uplink (UL) multiple-input multiple-output (MIMO), downlink MU-MIMO, etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
The downlink MU-MIMO technique, which is an advanced multi-antenna technique, can be used in an eNB to simultaneously transmit independent data to a plurality of UEs in the same frequency resources. A precoder is constructed to implement the downlink MU-MIMO technique. In other words, the construction of the precoder is based on feedback information from the UEs and regarded as an essential key to implement the downlink MU-MIMO technique. Therefore, each of the UEs should respectively send its favorite precoder index as well as the channel quality indication (CQI), to assist the eNB to construct a precoder matrix for downlink MU-MIMO transmissions.
Since a potential interference may exist in a paired UE assigned in a downlink MU-MIMO transmission, a resultant performance may be degraded due to over-estimated CQI and inappropriate modulation and coding schemes (MCS) assignment, and/or unsuitable mode selection from single-user MIMO (SU-MIMO) mode and MU-MIMO mode. Therefore, the standardization of 3GPP LTE has provided several directions for MU-MIMO enhancements, such as additional feedback with a precoder matrix index (PMI), a best companion PMI (BCI) and a multi-user CQI (MU-CQI). Generally speaking, PMI represents the precoder to be applied at the eNB that maximizes the signal power with respect to the UE, and BCI represents the precoder for the paired UE at the eNB that causes least interference to the UE. In addition, MU-CQI is the CQI derived by taking interference from the paired UE into account.
Besides, a paper, “Y. Du, J. Tong, J Zhang and S. Liu, ‘Evaluation of PMI Feedback Schemes for MU-MIMO Pairing’, IEEE Systems Journal, vol. 4, No. 4, pp. 505-510, December 2010”, has provided a feedback method for a UE. In this paper, a codebook of the precoder is divided into several clusters, each cluster is consisted of multiple codebook entries, and an index of the best companion cluster (BCC) is as a feedback instead of the BCI. The index of the BCC corresponds to a precoder cluster that contains a desirable BCI, and the eNB treats all entries in the BCC as the feasible companion precoders that can be paired with the UE. Besides, the MU-CQI is reported based on the average among all candidate companion precoders in the BCC.
Since the number of the BCCs is smaller than the codebook size, the approach provides scheduling flexibility while reducing feedback overhead. However, the method in the paper cannot capture the true degradation in the MU-CQI and the codebook clustering is specified offline, so that the performance of the method would be dramatically deteriorated when some of the more advanced precoding techniques are introduced in the future wireless communication systems. Therefore, it is necessary to provide other feedback methods to optimize the construction of the precoder.